Hi... I think it stems from the stereotype of the anoying aunt who comes to visit all the time. Flow of course is her name, which is just the witty remark about the "flow" of your period. Where I come from (and when we were kids) we would say "Aunt flow is visiting from RED Deer again." (Red Deer is a town) Its so funny how universally this term is used! (the Aunt flow part that is!)